"The English word holy dates back to at least the 11th Century with the Old English word hālig, an adjective derived from hāl meaning whole and used to mean 'uninjured, sound, healthy, entire, complete’. The Scottish 'hale' (health, happiness and wholeness.) is the most complete modern form of this Old English root. The modern word 'health' is also derived from the Old English hal. As “wholeness”, holiness may be taken to indicate a state of religious completeness or perfection. The word holy in its modern form appears in Wyclif's Bible of 1382."

And here's some about the word holistic:

"1926, coined, along with holism, by Gen. J.C. Smuts (1870-1950), from Gk. holos "whole" (see safe (adj.)). In reference to the theory that regards nature as consisting of wholes. "

It makes sense that the word appears in the dictionary spelled both ways.